The present invention relates to an apparatus for measuring the voltage developing in an object of interest by utilizing the change in the polarization state of light which occurs in response to the voltage-induced change in the refractive index of an electro-optic material.
FIG. 5 shows the basic composition of a presently available voltage detector.
The apparatus shown in FIG. 5 includes a light source 76, a light modulator 70 and a photodetector 71. The light modulator 70 includes a polarizer 72, an analyzer 74 and an electro-optic material 73 whose refractive index changes in accordance with the voltage developing in an object to be measured 75. When light from the light source 76 is launched into the light modulator 70, it is first linearly polarized by the polarizer 72 to produce incident light IL to be launched into the electro-optic material 73. The polarization state of the incident light IL changes in response to the change in the refractive index of the electro-optic material 73 which occurs in accordance with the voltage developing in the object to be measured 75. The resulting transmitted light TL is launched into the analyzer 74 which extracts from TL a polarized component that is perpendicular to the component extracted in the polarizer 72, thereby producing signal light SL. The intensity of signal light SL from the analyzer 74 changes in accordance with the voltage developing in the object 75 to be measured, so that said voltage can be measured by detecting the change in the intensity of SL by means of the photodetector 71.
It is a general fact that light falling at the boundary of two media having different indices of refraction or light emerging therefrom is partly reflected at that boundary, and this phenomenon is generally referred to as Fresnel reflection. Considering the boundary between air and a medium having a refractive index of n.sub.0, the Fresnel reflectance R with respect to normal incidence light is expressed by: EQU R=[(n.sub.0 -1)/(n.sub.0 +1)].sup.2. (1)
If the electro-optic material 73 is made of the crystal of LiTaO.sub.3 having relatively large refractive index n.sub.0 of 2.18, the equation (1) shows that as much as about 14% reflection occurs at the boundary between air and the LiTaO.sub.3 crystal.
The result of Fresnel reflection occurring in the electro-optic material 73 is shown in FIG. 6. The incident light IL launched into the electro-optic material 73 is reflected both by the incident and exit surfaces of the material 73 to produce transmitted light T.sub.1 which accounts for about 74% of incident light IL. Considering the transmitted light T.sub.2 which is produced as a result of multiple reflection within the electro-optic material 73, the overall proportion of the incident light IL present in the transmitted light TL is about 75.4%. As a result, even if the voltage developing in the object to be measured is at such a value that the signal light SL has a maximum intensity, its maximum intensity S.sub.max is significantly lowered compared with the case where zero Fresnel reflection is assumed (namely, 100% of the incident light IL is transmitted). It should be mentioned here that such a drop in the maximum intensity of SL is further increased by the Fresnel reflection that occurs in optical systems other than the electro-optic material 73 such as the polarizer 72 and the analyzer 74.
It should also be mentioned that the transmitted light T.sub.1 which has not experienced multiple reflection and the transmitted light T.sub.2 which is produced after multiple reflection have traveled through the electro-optic material 73 over different lengths of optical path. As a result, T.sub.2 differs from T.sub.1 in the state of polarization, making it impossible to reduce the minimum intensity S.sub.min of signal light SL to the ideal value, namely "0". The intensity of signal light SL assumes a minimum value S.sub.min when the voltage to be measured is such that the polarization state of transmitted light T.sub.1 prevents itself from being transmitted through the analyzer 74. However, even at such a voltage, part of the transmitted light T.sub.2 (whose intensity is about 1.8% of that of T.sub.1 for the case where the electro-optic material 73 is made of a LiTaO.sub.3 crystal) travels through the analyzer 74 and the minimum intensity S.sub.min of SL generally assumes a finite value which is not "0". Such a decrease in the maximum intensity S.sub.max signal light and an increase in its minimum intensity S.sub.min, both being due to Fresnel reflection, have caused a reduction in the extinction ratio (i.e. S.sub.max /S.sub.min) of signal light SL to a relatively low value of about 100. When the extinction ratio of the signal light SL drops, the dynamic range of voltage measurements which rely upon the detection of the intensity of signal light SL from the light modulator 70 by means of the light detector 71 is inevitably reduced because of the inability to attain a high modulation factor by means of lowering the optical bias point of the light modulator 70.
Another problem associated with the prior art is that the transmitted light T.sub.2 whose polarization state differs from that of T.sub.1 is contained in the signal light SL, so that the precision of measurement is reduced.
In order to compensate for the spontaneous birefringence that occurs within the electro-optic material 73, systems have been proposed that further include a Babinet compensator or a compensating electro-optic material that is as thick as the electro-optic material 73 and whose optic axis is inclined by 90 degrees with respect to that of the electro-optic material 73. But the above-described problems in association with Fresnel reflection have also occurred in such systems.